Certain components in the blood absorb light more strongly at different wavelengths. For example, oxyhemoglobin absorbs light more strongly in the infrared region then in the red region. Therefore, highly oxygenated blood having a high concentration of oxyhemoglobin will tend to have a high ratio of optical transmissivity in the infrared region. The ratio of transmissivities of the blood at red and infrared wavelengths can be employed in calculating oxygen saturation of the blood.
This principle has been used in oximeters for monitoring oxygen saturation of the blood, as for example, in patients undergoing surgery. Oximeters for this purpose may include a red and infrared light emitting diode together with a photodetector. The oximeter is typically clamped to an appendage of the patient's body, such as an ear or finger. The oximeter directs a beam of red and infrared light of known frequency and wavelength into the appendage. A sensor on the other end of the appendage receives the diffused light. Knowing the change in wavelength, frequency, and intensity of the diffused light beam, the oximeter can quickly determine the oxygen saturation level of the patient.
The diffused light signal received by the photodetector is an analog signal which includes both an AC and DC component. The diffused light signal includes an AC component which reflects the varying optical absorption of the blood due to variance in the volume of the blood due to the pulsatile flow of blood in the body. The diffused signal also includes an invariant or DC component related to other absorption, such as absorption by tissues other than blood in the body structure.
The diffused analog light signal is converted into a digital signal. The digital representation is used by the oximeter system microprocessor for processing the oxygen saturation level. Because calculation of the oxygen saturation is critical for determining the status of the patient, a high degree of accuracy is required for the digital representation of the diffused analog light signal. A problem in converting from an analog to a digital representation of the signal is that the DC component is so much larger than the AC component of the diffused signal. To encompass both the AC and DC components of the entire diffused signal requires using a 16 bit A/D converter. Using a smaller A/D converter, for example an 8 bit A/D converter, would cut off the least significant bits of the signal, namely the AC component of the signal. Receiving an accurate representation of the AC component is critical, since it is the AC component of the diffused signal which reflects the oxygen absorption.
Because of the degree of precision necessary to accurately reflect the AC component of the signal, currently available oximeter systems typically uses a 16-bit A/D converter. A 16 bit A/D converter may be four times as expensive as currently available microprocessors which include an 8 bit A/D converter and a pulse width modulator in a single chip. Since 8 bit does not give a sufficient degree of accuracy to encompass for oxygen absorption measurements, a combined 8 bit microprocessor A/D converter chips cannot be used. A separate 16 bit A/D converter must be used.
In addition to increasing the costs of the oximeter system, using a separate 16 bit A/D converter increases the size and power consumption of the system. Adding a separate 16 bit A/D converter adds to the size of the measurement system. Because the oximeter measurement system is connected to a patient and because the system is often moved between patient rooms, compactness of size of the pulse oximeter measurement system is highly desirable.
The digital representation of the diffused signal is used by the oximeter system microprocessor to calculate oxygen saturation level in the blood of a patient. The Ratio of Ratios, a variable used in calculating the oxygen saturation level, is typically calculated by taking the natural logarithm of the ratio of the peak value of the infrared signal divided by valley measurement of the red signal. The aforementioned value is then divided by the natural logarithm of the ratio of the peak value of the red signal divided by the value of valley measurement of the infrared signal.
The diffused signal is sampled several times during each period to determine the peak and valley measurement for each period of the waveform. In calculating the Ratio of Ratios, the peak value is assumed to be the high sample value during the period of the waveform. The valley measurement is assumed to be the low measured value. Although this method leads to a good estimate of the variable R, taking the peak and valley measurements over the entire time interval is prone to error since the sampling is taken between a single pair of points. This ignores variation in the signal between different pulses during the measured time interval.
A problem with choosing a single peak and valley measurement during a sampling interval, is corruption of the measurement by system noise. For example, patient motion of the oximeter during the sampling period may cause drift in the AC component of the signal. Also, ambient light or electrical noise may increase system noise. If the added noise on the pulse creates a false peak or valley measurement during the sampling interval, this will cause an incorrect value for the Ratio of Ratios. An inexpensive, noise insensitive oximeter measurement system is needed.